Xenogears 2.0 exists but it really doesn't do much for that sort of project outside of undubbing the cutscenes and restoring button combinations. It may or may not make a good foundation to start with if you ask though, unless a retranslation of the JP version comes with the restoration.
I'm sure that considering how much was left out, Xenogears has stuff in about the same state as notable unfinished PC RPGs (Troika's stuff anyone?) where all but the finishing touches are there for a lot of stuff, just dummied out. Have there been any attempts across the ocean? There's been multiple attempts in Japan to bring SaGa Frontier's missing content somewhere in any rate, so it's not unheard of (though the SaGa games have a pretty big hacker following, just look at RS3).
But the thing is, taking into account all that was planned for Xenogears... is it really worth? Even if we restored all that was intended in the game, I think it would be still be a cliffhanger in a way.